1. Field of the Invention
The present invention relates to a controller of an injection molding machine, particularly a controller to perform a stable switch from an injection process to a pressure-holding process.
2. Description of Related Art
In the molding by an injection molding machine, resin is injected to fill a cavity of a mold (injection process) and then the pressure exerted is kept at a predetermined pressure value for a predetermined time period (pressure-holding process). Generally, whether or not the cavity is filled with resin is determined by the position of an injection screw or by detecting an actual value for injection pressure, since when the cavity is filled with resin, the injection pressure increases. When the injection screw reaches a predetermined position or when the detected value for injection pressure is equal to or greater than a predetermined pressure value, control is performed to switch speed control on the injection screw in the injection process to pressure control for the pressure-holding process.
It is ideal if the pressure changes to a predetermined pressure-holding pressure (pressure at which the pressure should be kept in pressure holding) immediately when the injection process is switched to the pressure-holding process. Actually, however, the pressure does not immediately change to the predetermined pressure-holding pressure but reaches the predetermined pressure-holding pressure with a delay. Further, even when the pressure is changed to the pressure-holding pressure at the time a pressure value equal to a predetermined switch pressure (pressure with which the switch from the injection process to the pressure-holding process should be started) is detected, the pressure overshoots, namely increases beyond the predetermined switch pressure to take a peak value.
This peak pressure is exerted on the resin in the cavity of the mold and affects the quality of a molded article.
As a technique for keeping the peak pressure within a predetermined pressure range in the instance in which the switch from the injection process to the pressure-holding process is performed on the basis of the position of the injection screw, it is known to detect the resin pressure at the time the switch is performed and adjust the position which the injection screw takes when the switch is performed or when the measurement ends so that the pressure value detected is within the predetermined pressure range (see JP 7-9514A).
Further, an invention is known in which a time period Tpd taken for the pressure to change from a current detected value to a predetermined maximum injection pressure is obtained from a rate of increase of the injection pressure, which is obtained by differentiating the expression for the pressure detected by a pressure detector; a deceleration time period Tvd is obtained from the deceleration characteristic of an actuator and a predetermined injection speed; deceleration is started when the deceleration time period Tvd is equal to or greater than the time period Tpd taken for the pressure to reach the predetermined maximum injection pressure; a time period T1 taken for the pressure to increase from a predetermined pressure-holding pressure to the maximum injection pressure is measured; and the pressure is decreased from the maximum injection pressure to the pressure-holding pressure according to a pressure-decreasing pattern having a gradient which is an integer times the gradient of the measured time period T1 (see JP 7-1522A).
When the cavity of the mold is filled with resin in the injection process and the injection process is switched to the pressure-holding process, the pressure takes a peak value (maximum pressure) and the resin in the cavity experiences this peak pressure. Since the peak pressure affects the quality of a molded article, it is desirable to control the peak pressure accurately. In the above-mentioned inventions disclosed in JP 7-9514A and JP 7-1522A, a target for the peak pressure is predetermined and control is performed so that the peak pressure agrees with this target, however variation of the actual peak pressure is not taken into consideration.
In the control on the injection molding machine, the resin pressure is detected at every predetermined sampling period, and the switch from the injection process to the pressure-holding process is performed on the basis of the detected value for the resin pressure. The same is true of the inventions disclosed in JP 7-9514A and JP 7-1522A. The detected value for the resin pressure is subject to detection error due to the sampling period. Further, even when the speed control is immediately switched to the pressure control for pressure holding at the time a pressure value equal to the predetermined switch pressure is detected, the pressure overshoots and the peak pressure is not always the same.
FIGS. 2a and 2b are diagrams for explaining the variation of the peak pressure appearing when the injection process (speed control) is switched to the pressure-holding process (pressure control).
In FIGS. 2a and 2b, the horizontal axis represents time, and the vertical axis represents resin pressure. Vertical broken lines distributed along the axis representing time indicate sampling times at which the pressure is detected. The period between the sampling times is Δt. Pp is a pressure predetermined as a pressure value at which the pressure should be kept in pressure holding (referred to as “pressure-holding pressure”), P(v−p) is a resin pressure value predetermined as a criterion for determining whether to switch from injection to pressure holding (referred to as “injection/pressure-holding switch determining resin pressure”), and Ps is a pressure predetermined as a pressure value with which the switch should be started (referred to as “switch start pressure”). Pr is actual resin pressure, and Pr′ is a resin pressure value detected by a pressure sensor as a value that has reached or exceeded the injection/pressure-holding switch determining resin pressure P(v−p). Pc is a command value for pressure for the pressure control in the pressure-holding process.
Suppose that injection is performed, so that the resin pressure Pr increases and exceeds the injection/pressure-holding switch determining resin pressure P(v−p) a little after sampling time t1 as shown in FIG. 2a. At the sampling time t1, it is not recognized that the resin pressure Pr has reached the injection/pressure-holding switch determining resin pressure P(v−p). The fact that the resin pressure Pr has reached the injection/pressure-holding switch determining resin pressure P(v−p) is recognized at sampling time t2, namely one sampling period Δt after the sampling time t1 (detected resin pressure value Pr′). At this time, the switch to the pressure-holding process is performed and the pressure control is started, however the resin pressure overshoots and a peak pressure Pmax appears after the switch.
FIG. 2b shows an instance in which the resin pressure exceeds the injection/pressure-holding switch determining resin pressure P(v−p) after sampling time t1, a little before sampling time t2. At the sampling time t2, the fact that the resin pressure has reached the injection/pressure-holding switch determining resin pressure P(v−p) is recognized (detected resin pressure value Pr′). At this time, the switch to the pressure-holding process is performed and the pressure control is started. Also in this instance, the resin pressure overshoots and a peak pressure Pmax appears after the switch.
Comparison between FIGS. 2a and 2b shows that, in the instance of FIG. 2a, although the actual resin pressure Pr has reached the injection/pressure-holding switch determining resin pressure P(v−p), it is about one sampling period Δt later than in the instance of FIG. 2b that the fact that the actual resin pressure has reached the injection/pressure-holding switch determining resin pressure P(v−p) is recognized and the pressure-holding control is started. Hence, in the instance of FIG. 2a, after the actual resin pressure has reached the injection/pressure-holding switch determining resin pressure P(v−p), the pressure-holding control is started about one sampling period Δt later than in the instance of FIG. 2b, so that the amount of overshooting is larger and the peak pressure is greater than in the instance of FIG. 2b, correspondingly. Thus, as shown in FIG. 2, there is a variation ΔPmax of the peak pressure.
As stated above, regarding the time at which the pressure that has reached the injection/pressure-holding switch determining resin pressure P(v−p) is detected, there is a difference corresponding to one sampling period Δt at the most, and the peak pressure vanes corresponding to this difference.